1. Field of the Invention
The present invention relates to a wireless communication device, a wireless communication system, and a receiving circuit.
2. Description of the Related Art
An example of a receiver for a wireless local area network adopts a direct conversion method or a sliding IF (Intermediate Frequency) for downsizing or a low-power consumption.
FIG. 9A illustrates an exemplary structure of the receiver of the direct conversion method. The down conversion method down-converts a local signal having the same frequency (for example, 2.4 GHz) as that of a carrier signal and demodulates to a baseband signal.
FIG. 9B illustrates an exemplary structure of the receiver of the sliding IF method. The sliding IF method down-converting a local signal having a frequency two thirds of the frequency of a carrier signal, and thereafter down-converts a local signal having a half frequency of the frequency of the above local signal having the frequency two thirds of the frequency of the carrier signal, and demodulates to a baseband signal.
In both of the above methods, a phase locked loop (PLL) circuit or the like as a local signal source is necessary. A voltage controlled oscillator (VCO) included in the PLL circuit ordinarily has a structure where a LC oscillator having an inductor is used in a GHz band. Because the size of the inductor is large, a VCO having an oscillating frequency higher than the carrier frequency may be used for downsizing and a lower cost. However, there is a limit in the downsizing and the low-power consumption.
Further, in the example of FIGS. 9A and 9B, many components such as low noise amplifiers (LNA) 901, 902, 909, and 910, mixers 903, 904, 911, 912, 913, and 914 for converting the frequency, and filters 905, 906, 907, and 908 are necessary.
Meanwhile, there is known a technique wherein a phase shift keying (PSK) modulation is converted to an amplitude shift keying (ASK) modulation using an argument divider (see, for example, Japanese National Publication of International Patent Application No. 2005-521298). Further, there is known a technique wherein the argument divider such as an injection locked oscillator (ILO) and an analog multiplier are used to convert from a signal subjected to the PSK modulation to a baseband signal (see, for example, U.S. Pat. No. 7,095,274). An exemplary block chart of a binary phase shift keying (BPSK) modulation is illustrated in FIG. 10.
Referring to FIG. 10, a received signal having a frequency 2f and subjected to the BPSK modulation is divided into two signals by an electric power distribution device 1001 and the divided signals are input into argument dividers 1002 and 1003 such as an ILO. The argument dividers 1002 and 1003 output a signal having a frequency f being a half frequency of the frequency of the received signal when the received signal is input. Further, the phase of the output signal of the argument divider 1003 is changed so as to increase by a phase of 90 degrees when the phase of the received signal subjected to the BPSK modulation advances by 180 degrees. On the other hand, the phase of the output signal of the argument divider 1003 is changed so as to decrease by a phase of 90 degrees when the phase of the received signal subjected to the BPSK modulation advances by 180 degrees. Output signals of the argument dividers 1002 and 1003 are multiplied by the analog multiplier (a mixer) 1004 and caused to pass through a lowpass filter 1005 to form an output signal. In the example of FIG. 10 also, the mixer (the analog multiplier 1004) and the filter (the lowpass filter 1005) are included in a receiving circuit.
By the structure illustrated in FIG. 10, the number of parts of the receiver can be decreased. However, an ordinary wireless LAN device needs to deal with multiple modulation methods such as direct sequence spread spectrum (DSSS) or orthogonal frequency division multiplexing (OFDM). Therefore, the demodulation circuit disclosed in U.S. Pat. No. 7,095,274 is limited to a use for demodulation of BPSK and is not used for an ordinary wireless LAN device.
By the downsizing and the low-power consumption in the wireless LAN device, a versatile technique of the wireless LAN can be utilized in various fields. However, because a conventional wireless LAN device has a structure including multiple mixers, multiple filters, and so on, there is difficulty to attain the downsizing or the low-power consumption.